It's not necessarily malicious. It could be someone checking out your other posts to see where you're coming from, and then finding out that in turns out they disagree with all of your recent comments.
I agree, but that would probably be rare, especially across threads. The culture here for a while (HN in its "prime") was that if something merited a downvote, then it merited a comment articulating the dissent. We didn't have too many trolls (a few, but not many), so most of the time it was better to stimulate the discussion by posting your explicit disagreements.
In fact, it was common to upvote people who presented an interesting opinion even if you disagreed with it.
Now it's common to see someone downvoted into the gray without a single response. In short, if somebody has to ask "why the downvotes?" we're doing it wrong. (Presumably trolls know why they are getting downvoted.)
While that's true, it does seem like a pattern. I've been downvoted in recent days for offering someone praise for a sharing something useful. I can't help but shake my head as I know it's someone who doesn't like something I said on another post and is following me around.
